Transport Area Working Group M. Larsen
(tsvwg) TietoEnator
Internet-Draft F. Gont
Intended status: BCP UTN/FRH
Expires: August 19, 2010 February 15, 2010
Transport Protocol Port Randomization Recommendationsdraft-ietf-tsvwg-port-randomization-06
Abstract
Recently, awareness has been raised about a number of "blind" attacks
that can be performed against the Transmission Control Protocol (TCP)
and similar protocols. The consequences of these attacks range from
throughput-reduction to broken connections or data corruption. These
attacks rely on the attacker's ability to guess or know the five-
tuple (Protocol, Source Address, Destination Address, Source Port,
Destination Port) that identifies the transport protocol instance to
be attacked. This document describes a number of simple and
efficient methods for the selection of the client port number, such
that the possibility of an attacker guessing the exact value is
reduced. While this is not a replacement for cryptographic methods
for protecting the transport-protocol instance, the described port
number obfuscation algorithms provide improved security/obfuscation
with very little effort and without any key management overhead. The
algorithms described in this document are local policies that may be
incrementally deployed, and that do not violate the specifications of
any of the transport protocols that may benefit from them, such as
TCP, UDP, UDP-lite, SCTP, DCCP, and RTP (provided the RTP application
explicitly signals the RTP and RTCP port numbers).
Status of this Memo
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The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on August 19, 2010.
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Internet-Draft Port Randomization Recommendations February 20101. Introduction
Recently, awareness has been raised about a number of "blind" attacks
(i.e., attacks that can be performed without the need to sniff the
packets that correspond to the transport protocol instance to be
attacked) that can be performed against the Transmission Control
Protocol (TCP) [RFC0793] and similar protocols. The consequences of
these attacks range from throughput-reduction to broken connections
or data corruption [I-D.ietf-tcpm-icmp-attacks] [RFC4953] [Watson].
All these attacks rely on the attacker's ability to guess or know the
five-tuple (Protocol, Source Address, Source port, Destination
Address, Destination Port) that identifies the transport protocol
instance to be attacked.
Services are usually located at fixed, 'well-known' ports [IANA] at
the host supplying the service (the server). Client applications
connecting to any such service will contact the server by specifying
the server IP address and service port number. The IP address and
port number of the client are normally left unspecified by the client
application and thus chosen automatically by the client networking
stack. Ports chosen automatically by the networking stack are known
as ephemeral ports [Stevens].
While the server IP address and well-known port and the client IP
address may be known by an attacker, the ephemeral port of the client
is usually unknown and must be guessed.
This document describes a number of algorithms for the selection of
ephemeral port numbers, such that the possibility of an off-path
attacker guessing the exact value is reduced. They are not a
replacement for cryptographic methods of protecting a transport-
protocol instance such as IPsec [RFC4301], the TCP MD5 signature
option [RFC2385], or the TCP Authentication Option
[I-D.ietf-tcpm-tcp-auth-opt]. For example, they do not provide any
mitigation in those scenarios in which the attacker is able to sniff
the packets that correspond to the transport protocol instance to be
attacked. However, the proposed algorithms provide improved
obfuscation with very little effort and without any key management
overhead.
The mechanisms described in this document are local modifications
that may be incrementally deployed, and that do not violate the
specifications of any of the transport protocols that may benefit
from them, such as TCP [RFC0793], UDP [RFC0768], SCTP [RFC4960], DCCP
[RFC4340], UDP-lite [RFC3828], and RTP [RFC3550] (provided the RTP
application explicitly signals the RTP and RTCP port numbers with
e.g.[RFC3605]).
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Since these mechanisms are obfuscation techniques, focus has been on
a reasonable compromise between the level of obfuscation and the ease
of implementation. Thus the algorithms must be computationally
efficient, and not require substantial state.
We note that while the technique of mitigating "blind" attacks by
obfuscating the ephemeral port selection is well-known as "port
randomization", the goal of the algorithms described in this document
is to reduce the chances of an attacker guessing the ephemeral ports
selected for new transport protocol instances, rather than to
actually produce mathematically random sequences of ephemeral ports.
Throughout this document we will use the term "transport-protocol
instance" as a general term to refer to an instantiation of a
transport protocol (e.g, a "connection" in the case of connection-
oriented transport protocols) and the term "instance-id" as a short-
handle to refer to the group of values that identify a transport-
protocol instance (e.g., in the case of TCP, the five-tuple
{Protocol, IP Source Address, TCP Source Port, IP Destination
Address, TCP Destination Port}).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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Internet-Draft Port Randomization Recommendations February 20102. Ephemeral Ports2.1. Traditional Ephemeral Port Range
The Internet Assigned Numbers Authority (IANA) assigns the unique
parameters and values used in protocols developed by the Internet
Engineering Task Force (IETF), including well-known ports [IANA].
IANA has reserved the following use of the 16-bit port range of TCP
and UDP:
o The Well Known Ports, 0 through 1023.
o The Registered Ports, 1024 through 49151
o The Dynamic and/or Private Ports, 49152 through 65535
The dynamic port range defined by IANA consists of the 49152-65535
range, and is meant for the selection of ephemeral ports.
2.2. Ephemeral port selection
As each communication instance is identified by the five-tuple
{protocol, local IP address, local port, remote IP address, remote
port}, the selection of ephemeral port numbers must result in a
unique five-tuple.
Selection of ephemeral ports such that they result in unique
instance-id's (five-tuples) is handled by some implementations by
having a per-protocol global 'next_ephemeral' variable that is equal
to the previously chosen ephemeral port + 1, i.e. the selection
process is:
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/* Initialization at system boot time. Could be random */
next_ephemeral = min_ephemeral;
/* Ephemeral port selection function */
count = max_ephemeral - min_ephemeral + 1;
do {
port = next_ephemeral;
if (next_ephemeral == max_ephemeral) {
next_ephemeral = min_ephemeral;
} else {
next_ephemeral++;
}
if (five-tuple is unique)
return port;
count--;
} while (count > 0);
return ERROR;
Figure 1
This algorithm works adequately provided that the number of
transport-protocol instances (for a each transport protocol) that
have a life-time longer than it takes to exhaust the total ephemeral
port range is small, so that collisions of instance-id's are rare.
However, this method has the drawback that the 'next_ephemeral'
variable and thus the ephemeral port range is shared between all
transport-protocol instances and the next ports chosen by the client
are easy to predict. If an attacker operates an "innocent" server to
which the client connects, it is easy to obtain a reference point for
the current value of the 'next_ephemeral' variable. Additionally, if
an attacker could force a client to periodically establish e.g., a
new TCP connection to an attacker controlled machine (or through an
attacker observable routing path), the attacker could subtract
consecutive source port values to obtain the number of outgoing TCP
connections established globally by the target host within that time
period (up to wrap-around issues and instance-id collisions, of
course).
2.3. Collision of instance-id's
While it is possible for the ephemeral port selection algorithm to
verify that the selected port number results in a instance-id that is
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not currently in use by that system, the resulting instance-id may
still be in use at a remote system. For example, consider a scenario
in which a client establishes a TCP connection with a remote web
server, and the web server performs the active close on the
connection. While the state information for this connection will
disappear at the client side (that is, the connection will be moved
to the fictional CLOSED state), the instance-id will remain in the
TIME-WAIT state at the web server for 2*MSL (Maximum Segment
Lifetime). If the same client tried to create a new incarnation of
the previous connection (that is, a connection with the same
instance-id as the one in the TIME_WAIT state at the server), an
instance-id "collision" would occur. The effect of these collisions
range from connection-establishment failures to TIME-WAIT state
assassination (with the potential of data corruption) [RFC1337]. In
scenarios in which a specific client establishes TCP connections with
a specific service at a server, these problems become evident.
Therefore, an ephemeral port selection algorithm should ideally
minimize the rate of instance-id collisions.
A simple approach to minimize the rate of these collisions would be
to choose port numbers incrementally, so that a given port number
would not be reused until the rest of the port numbers in ephemeral
port range have been used for a transport protocol instance.
However, if a single global variable were used to keep track of the
last ephemeral port selected, ephemeral port numbers would be
trivially predictable, thus making it easier for an off-path attacker
to "guess" the instance-id in use by a target transport-protocol
instance. Section 3.3.3 and Section 3.3.4 describe algorithms that
select port numbers incrementally, while still making it difficult
for an off-path attacker to predict the ephemeral ports used for
future transport-protocol instances.
A simple but inefficient approach to minimize the rate of collisions
of instance-id's would be, e.g. in the case of TCP, for both end-
points of a TCP connection to keep state about recent connections
(e.g., have both end-points end up in the TIME-WAIT state).
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Internet-Draft Port Randomization Recommendations February 20103. Obfuscating the Ephemeral Ports3.1. Characteristics of a good ephemeral port obfuscation algorithm
There are a number of factors to consider when designing an algorithm
for selecting ephemeral ports, which include:
o Minimizing the predictability of the ephemeral port numbers used
for future transport-protocol instances.
o Minimizing collisions of instance-id's
o Avoiding conflict with applications that depend on the use of
specific port numbers.
Given the goal of improving the transport protocol's resistance to
attack by obfuscation of the instance-id, it is key to minimize the
predictability of the ephemeral ports that will be selected for new
transport-protocol instances. While the obvious approach to address
this requirement would be to select the ephemeral ports by simply
picking a random value within the chosen port number range, this
straightforward policy may lead to collisions of instance-id's, which
could lead to the interoperability problems (e.g., delays in the
establishment of new connections, failures in connection-
establishment, or data corruption) discussed in Section 2.3. As
discussed in Section 1, it is worth noting that while the technique
of mitigating "blind" attacks by obfuscating the ephemeral port
election is well-known as "port randomization", the goal of the
algorithms described in this document is to reduce the chances of an
attacker guessing the ephemeral ports selected for new transport-
protocol instances, rather than to actually produce sequences of
mathematically random ephemeral port numbers.
It is also worth noting that, provided adequate algorithms are in
use, the larger the range from which ephemeral pots are selected, the
smaller the chances of an attacker are to guess the selected port
number.
In scenarios in which a specific client establishes transport-
protocol instances with a specific service at a server, the problems
described in Section 2.3 become evident. A good algorithm to
minimize the collisions of instance-id's would consider the time a
given five-tuple was last used, and would avoid reusing the last
recently used five-tuples. A simple approach to minimize the rate of
collisions would be to choose port numbers incrementally, so that a
given port number would not be reused until the rest of the port
numbers in the ephemeral port range have been used for a transport
protocol instance. However, if a single global variable were used to
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keep track of the last ephemeral port selected, ephemeral port
numbers would be trivially predictable.
It is important to note that a number of applications rely on binding
specific port numbers that may be within the ephemeral ports range.
If such an application was run while the corresponding port number
was in use, the application would fail. Therefore, ephemeral port
selection algorithms avoid using those port numbers.
Port numbers that are currently in use by a TCP in the LISTEN state
should not be allowed for use as ephemeral ports. If this rule is
not complied with, an attacker could potentially "steal" an incoming
connection to a local server application by issuing a connection
request to the victim client at roughly the same time the client
tries to connect to the victim server application [CPNI-TCP]
[I-D.gont-tcp-security]. If the SYN segment corresponding to the
attacker's connection request and the SYN segment corresponding to
the victim client "cross each other in the network", and provided the
attacker is able to know or guess the ephemeral port used by the
client, a TCP simultaneous open scenario would take place, and the
incoming connection request sent by the client would be matched with
the attacker's socket rather than with the victim server
application's socket.
It should be noted that most applications based on popular
implementations of the TCP API (such as the Sockets API) perform
"passive opens" in three steps. Firstly, the application obtains a
file descriptor to be used for inter-process communication (e.g., by
issuing a socket() call). Secondly, the application binds the file
descriptor to a local TCP port number (e.g., by issuing a bind()
call), thus creating a TCP in the fictional CLOSED state. Thirdly,
the aforementioned TCP is put in the LISTEN state (e.g., by issuing a
listen() call). As a result, with such an implementation of the TCP
API, even if port numbers in use for TCPs in the LISTEN state were
not allowed for use as ephemeral ports, there is a window of time
between the second and the third steps in which an attacker could be
allowed to select a port number that would be later used for
listening to incoming connections. Therefore, these implementations
of the TCP API should enforce a stricter requirement for the
allocation of port numbers: port numbers that are in use by a TCP in
the LISTEN or CLOSED states should not be allowed for allocation as
ephemeral ports [CPNI-TCP] [I-D.gont-tcp-security].
The aforementioned issues do not affect SCTP, since most SCTP
implementations do not allow a socket to be bound to the same port
number unless a specific socket option (SCTP_REUSE_PORT) is issued on
the socket (i.e., this behavior needs to be explititly allowed
beforehand). An example of a typical SCTP socket API can be found in
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[I-D.ietf-tsvwg-sctpsocket].
DCCP is not affected is not affected by the exploitation of
"simultaneous opens" to ""steal" incoming connections, as the server
and the client state machines are different [RFC4340]. However, it
may be affected by the vector involving binding a more specific
socket. As a result, those tuples {local IP address, local port,
Service Code} that are in use by a local socket should not be allowed
for allocation as ephemeral ports.
3.2. Ephemeral port number range
As mentioned in Section 2.1, the dynamic ports consist of the range
49152-65535. However, ephemeral port selection algorithms should use
the whole range 1024-49151.
Since this range includes ports numbers assigned by IANA, this may
not always be possible, though. A possible workaround for this
potential problem would be to maintain a local list of the port
numbers that should not be allocated as ephemeral ports. Thus,
before allocating a port number, the ephemeral port selection
function would check this list, avoiding the allocation of ports that
may be needed for specific applications.
Ephemeral port selection algorithms SHOULD use the largest possible
port range, since this improves obfuscation.
3.3. Ephemeral Port Obfuscation Algorithms
Ephemeral port selection algorithms SHOULD obfuscate the allocation
of their ephemeral ports, since this helps to mitigate a number of
attacks that depend on the attacker's ability to guess or know the
five-tuple that identifies the transport protocol instance to be
attacked.
The following subsections describe a number of algorithms that could
be implemented in order to obfuscate the selection of ephemeral port
numbers.
3.3.1. Algorithm 1: Simple port randomization algorithm
In order to address the security issues discussed in Section 1 and
Section 2.2, a number of systems have implemented simple ephemeral
port number randomization, as follows:
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/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
next_ephemeral = min_ephemeral + (random() % num_ephemeral);
count = num_ephemeral;
do {
if(resulting five-tuple is unique)
return next_ephemeral;
if (next_ephemeral == max_ephemeral) {
next_ephemeral = min_ephemeral;
} else {
next_ephemeral++;
}
count--;
} while (count > 0);
return ERROR;
Figure 2
We will refer to this algorithm as 'Algorithm 1'.
Note: "random()" is a function that returns a pseudo-random unsigned
interger number in the range 0-65535 (it may return values larger
than 65535, as is the case with the "random()" C-language function).
Since the initially chosen port may already be in use with identical
IP addresses and server port, the resulting five-tuple might not be
unique. Therefore, multiple ports may have to be tried and verified
against all existing transport-protocol instances before a port can
be chosen.
Web proxy servers, NAPTs [RFC2663], and other middle-boxes aggregate
multiple peers into the same port space and thus increase the
population of used ephemeral ports, and hence the chances of
collisions of instance-id's. However, [Allman] has shown that at
least in the network scenarios used for measuring the collision
properties of the algorithms described in this document, the
collision rate resulting from the use of the aforementioned middle-
boxes is nevertheless very low.
Since this algorithm performs a completely random port selection
(i.e., without taking into account the port numbers previously
chosen), it has the potential of reusing port numbers too quickly,
thus possibly leading to collisions of instance-id's. Even if a
given five-tuple is verified to be unique by the port selection
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algorithm, the five-tuple might still be in use at the remote system.
In such a scenario, a connection request could possibly fail
([Silbersack] describes this problem for the TCP case).
This algorithm selects ephemeral port numbers randomly and thus
reduces the chances of an attacker of guessing the ephemeral port
selected for a target transport-protocol instance. Additionally, it
prevents attackers from obtaining the number of outgoing transport-
protocol instances (e.g., TCP connections) established by the client
in some period of time.
3.3.2. Algorithm 2: Another simple port randomization algorithm
The following pseudo-code illustrates another algorithm for selecting
a random port number, in which in the event a local instance-id
collision is detected, another port number is selected randomly:
/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
next_ephemeral = min_ephemeral + (random() % num_ephemeral);
count = num_ephemeral;
do {
if(resulting five-tuple is unique)
return next_ephemeral;
next_ephemeral = min_ephemeral + (random() % num_ephemeral);
count--;
} while (count > 0);
return ERROR;
Figure 3
We will refer to this algorithm as 'Algorithm 2'. This algorithm
might be unable to select an ephemeral port (i.e., return "ERROR")
even if there are port numbers that would result in unique five-
tuples, when there are a large number of port numbers already in use.
However, the results in [Allman] have shown that in common scenarios,
one port choice is enough, and in most cases where more than one
choice is needed two choices suffice. Therefore, in those scenarios
this would not be problem.
3.3.3. Algorithm 3: Simple hash-based algorithm
We would like to achieve the port reuse properties of the traditional
BSD port selection algorithm (described in Section 2.2), while at the
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same time achieve the obfuscation properties of Algorithm 1 and
Algorithm 2.
Ideally, we would like a 'next_ephemeral' value for each set of
(local IP address, remote IP addresses, remote port), so that the
port reuse frequency is the lowest possible. Each of these
'next_ephemeral' variables should be initialized with random values
within the ephemeral port range and would thus separate the ephemeral
port space of the transport-protocol instances on a "per destination
end-point" basis (this "separation of the ephemeral port space" means
that transport-protocol instances with different remote end-points
will not have different sequences of port numbers; i.e., wil not be
part of the same ephemeral port sequence as in the case of the
traditional BSD ephemeral port selection algorithm). Since we do not
want to maintain in memory all these 'next_ephemeral' values, we
propose an offset function F(), that can be computed from the local
IP address, remote IP address, remote port and a secret key. F()
will yield (practically) different values for each set of arguments,
i.e.:
/* Initialization at system boot time. Could be random. */
next_ephemeral = 0;
/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
offset = F(local_IP, remote_IP, remote_port, secret_key);
count = num_ephemeral;
do {
port = min_ephemeral +
(next_ephemeral + offset) % num_ephemeral;
next_ephemeral++;
if(resulting five-tuple is unique)
return port;
count--;
} while (count > 0);
return ERROR;
Figure 4
We will refer to this algorithm as 'Algorithm 3'.
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In other words, the function F() provides a "per destination end-
point" fixed offset within the global ephemeral port range. Both the
'offset' and 'next_ephemeral' variables may take any value within the
storage type range since we are restricting the resulting port in a
similar way as in the Algorithm 1 (described in Section 3.3.1). This
allows us to simply increment the 'next_ephemeral' variable and rely
on the unsigned integer to simply wrap-around.
The function F() should be a cryptographic hash function like MD5
[RFC1321]. The function should use both IP addresses, the remote
port and a secret key value to compute the offset. The remote IP
address is the primary separator and must be included in the offset
calculation. The local IP address and remote port may in some cases
be constant and not improve the ephemeral port space separation,
however, they should also be included in the offset calculation.
Cryptographic algorithms stronger than e.g. MD5 should not be
necessary, given that Algorithm #3 is simply an obfuscation
technique. The secret should be chosen as random as possible, see
[RFC4086] for recommendations on choosing secrets.
Note that on multiuser systems, the function F() could include user
specific information, thereby providing protection not only on a host
to host basis, but on a user to service basis. In fact, any
identifier of the remote entity could be used, depending on
availability an the granularity requested. With SCTP both hostnames
and alternative IP addresses may be included in the association
negotiation and either of these could be used in the offset function
F().
When multiple unique identifiers are available, any of these can be
chosen as input to the offset function F() since they all uniquely
identify the remote entity. However, in cases like SCTP where the
ephemeral port must be unique across all IP address permutations, we
should ideally always use the same IP address to get a single
starting offset for each association negotiation from a given remote
entity to minimize the possibility of collisions. A simple numerical
sorting of the IP addresses and always using the numerically lowest
could achieve this. However, since most protocols most likely will
report the same IP addresses in the same order in each association
setup, this sorting is most likely not necessary and the 'first one'
can simply be used.
The ability of hostnames to uniquely define hosts can be discussed,
and since SCTP always includes at least one IP address, we recommend
to use this as input to the offset function F() and ignore hostnames
chunks when searching for ephemeral ports.
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It should be noted that, as this algorithm uses a global counter
("next_ephemeral") for selecting ephemeral ports, if an attacker
could e.g., force a client to periodically establish a new TCP
connections to an attacker controlled machine (or through an attacker
observable routing path), the attacker could subtract consecutive
source port values to obtain the number of outgoing TCP connections
established globally by the target host within that time period (up
to wrap-around issues and 5-tuple collisions, of course).
3.3.4. Algorithm 4: Double-hash obfuscation algorithm
A tradeoff between maintaining a single global 'next_ephemeral'
variable and maintaining 2**N 'next_ephemeral' variables (where N is
the width of the result of F()) could be achieved as follows. The
system would keep an array of TABLE_LENGTH short integers, which
would provide a separation of the increment of the 'next_ephemeral'
variable. This improvement could be incorporated into Algorithm 3 as
follows:
/* Initialization at system boot time */
for(i = 0; i < TABLE_LENGTH; i++)
table[i] = random() % 65536;
/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
offset = F(local_IP, remote_IP, remote_port, secret_key1);
index = G(local_IP, remote_IP, remote_port, secret_key2);
count = num_ephemeral;
do {
port = min_ephemeral + (offset + table[index]) % num_ephemeral;
table[index]++;
if(resulting five-tuple is unique)
return port;
count--;
} while (count > 0);
return ERROR;
Figure 5
We will refer to this algorithm as 'Algorithm 4'.
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'table[]' could be initialized with mathematically random values, as
indicated by the initialization code in pseudo-code above. The
function G() should be a cryptographic hash function like MD5
[RFC1321]. It should use both IP addresses, the remote port and a
secret key value to compute a value between 0 and (TABLE_LENGTH-1).
Alternatively, G() could take as "offset" as input, and perform the
exclusive-or (xor) operation between all the bytes in 'offset'.
The array 'table[]' assures that successive transport-protocol
instances with the same remote end-point will use increasing
ephemeral port numbers. However, incrementation of the port numbers
is separated into TABLE_LENGTH different spaces, and thus the port
reuse frequency will be (probabilistically) lower than that of
Algorithm 3. That is, a new tranport-protocol instance with some
remote end-point will not necessarily cause the 'next_ephemeral'
variable corresponding to other end-points to be incremented.
It is interesting to note that the size of 'table[]' does not limit
the number of different port sequences, but rather separates the
*increments* into TABLE_LENGTH different spaces. The port sequence
will result from adding the corresponding entry of 'table[]' to the
variable 'offset', which selects the actual port sequence (as in
Algorithm 3). [Allman] has found that a TABLE_LENGTH of 10 can
result in an improvement over Algorithm 3. Further increasing the
TABLE_LENGTH will increase the obfuscation, and possibly further
decrease the collision rate.
An attacker can perform traffic analysis for any "increment space"
into which the attacker has "visibility", namely that the attacker
can force the client to establish a transport-protocol instance whose
G(offset) identifies the target "increment space". However, the
attacker's ability to perform traffic analysis is very reduced when
compared to the traditional BSD algorithm (described in Section 2.2)
and Algorithm 3. Additionally, an implementation can further limit
the attacker's ability to perform traffic analysis by further
separating the increment space (that is, using a larger value for
TABLE_LENGTH).
3.3.5. Algorithm 5: Random-increments port selection algorithm
[Allman] introduced another port obfuscation algorithm, which offers
a middle ground between the algorithms that select ephemeral ports
randomly (such as those described in Section 3.3.1 and
Section 3.3.2), and those that offer obfuscation but no randomization
(such as those described in Section 3.3.3 and Section 3.3.4). We
will refer to this algorithm as 'Algorithm 5'.
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Internet-Draft Port Randomization Recommendations February 2010
/* Initialization code at system boot time. */
next_ephemeral = random() % 65536; /* Initialization value */
N = 500; /* Determines the tradeoff */
/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
count = num_ephemeral;
do {
next_ephemeral = next_ephemeral + (random() % N) + 1;
port = min_ephemeral + (next_ephemeral % num_ephemeral);
if(resulting five-tuple is unique)
return port;
count--;
} while (count > 0);
return ERROR;
Figure 6
This algorithm aims at at producing a monotonically-increasing
sequence to prevent the collision of instance-id's, while avoiding
the use of fixed increments, which would lead to trivially-
predictable sequences. The value "N" allows for direct control of
the tradeoff between the level of obfuscation and the port reuse
frequency. The smaller the value of "N", the more linear the more
similar this algorithm is to the traditional BSD port selection
algorithm (described in Section 2.2. The larger the value of "N",
the more similar this algorithm is to the algorithm described in
Section 3.3.1 of this document.
When the port numbers wrap, there is the risk of collisions of
instance-id's. Therefore, "N" should be selecting according to the
following criteria:
o It should maximize the wrapping time of the ephemeral port space
o It should minimize collisions of instance-id's
o It should maximize obfuscation
Clearly, these are competing goals, and the decision of which value
of "N" to use is a tradeoff. Therefore, the value of "N" should be
configurable so that system administrators can make the tradeoff for
themselves.
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Internet-Draft Port Randomization Recommendations February 20103.4. Secret-key considerations for hash-based port obfuscation algorithms
Every complex manipulation (like MD5) is no more secure than the
input values, and in the case of ephemeral ports, the secret key. If
an attacker is aware of which cryptographic hash function is being
used by the victim (which we should expect), and the attacker can
obtain enough material (e.g. ephemeral ports chosen by the victim),
the attacker may simply search the entire secret key space to find
matches.
To protect against this, the secret key should be of a reasonable
length. Key lengths of 32 bits should be adequate, since a 32-bit
secret would result in approximately 65k possible secrets if the
attacker is able to obtain a single ephemeral port (assuming a good
hash function). If the attacker is able to obtain more ephemeral
ports, key lengths of 64 bits or more should be used.
Another possible mechanism for protecting the secret key is to change
it after some time. If the host platform is capable of producing
reasonable good random data, the secret key can be changed
automatically.
Changing the secret will cause abrupt shifts in the chosen ephemeral
ports, and consequently collisions may occur. That is, upon changing
the secret, the "offset" value (see Section 3.3.3 and Section 3.3.4)
used for each destination end-point will be different from that
computed with the previous secret, thus leading to the selection of a
port number recently used for connecting to the same end-point.
Thus the change in secret key should be done with consideration and
could be performed whenever one of the following events occur:
o The system is being bootstrapped.
o Some predefined/random time has expired.
o The secret has been used N times (i.e. we consider it insecure).
o There are few active transport protocol instances (i.e.,
possibility of collision is low).
o There is little traffic (the performance overhead of collisions is
tolerated).
o There is enough random data available to change the secret key
(pseudo-random changes should not be done).
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Internet-Draft Port Randomization Recommendations February 20103.5. Choosing an ephemeral port obfuscation algorithm
[Allman] is an empirical study of the properties of the algorithms
described in this document, which has found that all the algorithms
described in this document offer low collision rates -- at most 0.3%.
That is, in those network scenarios assessed by [Allman] all of the
algorithms described in this document perform well in terms of
collisions of instance-id's. However, these results may vary
depending on the characteristics of network traffic and the specific
network setup.
The algorithm described in Section 2.2 is the traditional ephemeral
port selection algorithm implemented in BSD-derived systems. It
generates a global sequence of ephemeral port numbers, which makes it
trivial for an attacker to predict the port number that will be used
for a future transport protocol instance. However, it is very
simple, and leads to a low port reuse frequency.
Algorithm 1 and Algorithm 2 have the advantage that they provide
actual randomization of the ephemeral ports. However, they may
increase the chances of port number collisions, which could lead to
the failure of a connection establishment attempt. [Allman] found
that these two algorithms show the largest collision rates (among all
the algorithms described in this document).
Algorithm 3 provides complete separation in local and remote IP
addresses and remote port space, and only limited separation in other
dimensions (see Section 3.4). However, implementations should
consider the performance impact of computing the cryptographic hash
used for the offset.
Algorithm 4 improves Algorithm 3, usually leading to a lower port
reuse frequency, at the expense of more processor cycles used for
computing G(), and additional kernel memory for storing the array
'table[]'.
Algorithm 5 offers middle ground between the simple randomization
algorithms (Algorithm 1 and Algorithm 2) and the hash-based
algorithms (Algorithm 3 and Algorithm 4). The upper limit on the
random increments (the value "N" in the pseudo-code included in
Section 3.3.5 controls the trade-off between randomization and port-
reuse frequency.
Finally, a special case that may preclude the utilization of
Algorithm 3 and Algorithm 4 should be analyzed. There exist some
applications that contain the following code sequence:
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Internet-Draft Port Randomization Recommendations February 2010
s = socket();
bind(s, IP_address, port = *);
Figure 7
In some BSD-derived systems, the call to bind() will result in the
selection of an ephemeral port number. However, as neither the
remote IP address nor the remote port will be available to the
ephemeral port selection function, the hash function F() used in
Algorithm 3 and Algorithm 4 will not have all the required arguments,
and thus the result of the hash function will be impossible to
compute. Transport protocols implementing Algorithm 3 or Algorithm 4
should consider using Algorithm 2 when facing the scenario just
described.
An alternative to this behavior would be to implement "lazy binding"
in response to the bind() call. That is, selection of an ephemeral
port would be delayed until, e.g., connect() or send() are called.
Thus, at that point the ephemeral port is actually selected, all the
necessary arguments for the hash function F() would be available, and
thus Algorithm 3 and Algorithm 4 could still be used in this
scenario. This algorithm has been implemented by Linux [Linux].
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Internet-Draft Port Randomization Recommendations February 20104. Port obfuscation and Network Address Port Translation (NAPT)
Network Address Port Translation (NAPT) translate both the network
address and transport-protocol port number, thus allowing the
transport identifiers of a number of private hosts to be multiplexed
into the transport identifiers of a single external address.
[RFC2663]
In those scenarios in which a NAPT is present between the two end-
points of transport-protocol instance, the obfuscation of the
ephemeral ports (from the point of view of the external network) will
depend on the ephemeral port selection function at the NAPT.
Therefore, NAPTs should consider obfuscating the ephemeral ports by
means of any of the algorithms discussed in this document. It should
be noted that in some network scenarios, a NAPT may naturally obscure
ephemeral port selections simply due to the vast range of services
with which it establishes connections and to the overall rate of the
traffic [Allman].
Section 3.5 provides guidance in choosing a port obfuscation
algorithm.
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Internet-Draft Port Randomization Recommendations February 20105. Security Considerations
Obfuscating ephemeral ports is no replacement for cryptographic
mechanisms, such as IPsec [RFC4301], in terms of protecting
transport-protocol instances against blind attacks.
An eavesdropper, which can monitor the packets that correspond to the
transport-protocol instance to be attacked could learn the IP
addresses and port numbers in use (and also sequence numbers etc.)
and easily perform an attack. Ephemeral port obfuscation does not
provide any additional protection against this kind of attacks. In
such situations, proper authentication mechanisms such as those
described in [RFC4301] should be used.
If the local offset function F() results in identical offsets for
different inputs, the port-offset mechanism proposed in this document
has no or reduced effect.
If random numbers are used as the only source of the secret key, they
must be chosen in accordance with the recommendations given in
[RFC4086].
If an attacker uses dynamically assigned IP addresses, the current
ephemeral port offset (Algorithm 3 and Algorithm 4) for a given five-
tuple can be sampled and subsequently used to attack an innocent peer
reusing this address. However, this is only possible until a re-
keying happens as described above. Also, since ephemeral ports are
only used on the client side (e.g. the one initiating the transport-
protocol communication), both the attacker and the new peer need to
act as servers in the scenario just described. While servers using
dynamic IP addresses exist, they are not very common and with an
appropriate re-keying mechanism the effect of this attack is limited.
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Internet-Draft Port Randomization Recommendations February 20106. IANA Considerations
There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.
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Internet-Draft Port Randomization Recommendations February 2010Appendix A. Survey of the algorithms in use by some popular
implementations
A.1. FreeBSD
FreeBSD 8.0 implements Algorithm 1, and in response to this document
now uses a 'min_port' of 10000 and a 'max_port' of 65535. [FreeBSD]
A.2. Linux
Linux 2.6.15-53-386 implements Algorithm 3. If the algorithm is
faced with the corner-case scenario described in Section 3.5,
Algorithm 1 is used instead [Linux].
A.3. NetBSD
NetBSD 5.0.1 does not obfuscate its ephemeral port numbers. It
selects ephemeral port numbers from the range 49152-65535, starting
from port 65535, and decreasing the port number for each ephemeral
port number selected [NetBSD].
A.4. OpenBSD
OpenBSD 4.2 implements Algorithm 1, with a 'min_port' of 1024 and a
'max_port' of 49151. [OpenBSD]
A.5. OpenSolaris
OpenSolaris 2009.06 implements Algorithm 1, with a 'min_port' of
32768 and a 'max_port' of 65535. [OpenSolaris]
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Internet-Draft Port Randomization Recommendations February 2010
o Included references and text on protocols other than TCP.
o Added the second variant of the simple port randomization
algorithm
o Reorganized the algorithms into different sections
o Miscellaneous editorial changes.
B.8. Changes from draft-larsen-tsvwg-port-randomization-01
o No changes. Draft resubmitted after expiration.
B.9. Changes from draft-larsen-tsvwg-port-randomization-00
o Fixed a bug in expressions used to calculate number of ephemeral
ports
o Added a survey of the algorithms in use by popular TCP
implementations
o The whole document was reorganized
o Miscellaneous editorial changes
B.10. Changes from draft-larsen-tsvwg-port-randomisation-00
o Document resubmitted after original document by M. Larsen expired
in 2004
o References were included to current WG documents of the TCPM WG
o The document was made more general, to apply to all transport
protocols
o Miscellaneous editorial changes
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